The present invention concerns signal analysis and pertains particularly to enhanced signal analysis in an amplitude versus frequency format.
A spectrum analyzer is a tunable frequency instrument that displays a portion of the frequency spectrum with amplitude of signals on the vertical axis and frequency on the horizontal axis on a screen. Spectrum analyzers can be useful when monitoring various types of transmission systems. Spectrum analyzers are typically used for analyzing optical or radio frequency transmissions. For example, wavelength division multiplexing (WDM) fiber optic transmission systems use between 4 and 500 laser signals to transmit information. Each laser signal resides within a specific frequency channel. The channel is defined by its center frequency, bandwidth, minimum and maximum signal power. A spectrum analyzer can be used to monitor the performance of each laser signal within the many channels of a WDM system.
When measuring a signal using a spectrum analyzer, the signal is displayed in the amplitude versus frequency format. The general technique used begins with a wide span, allowing the user to observe a number of signals (if present). To xe2x80x9czoom inxe2x80x9d on a desired signal, the user can use either the frequency center and span functions, or marker-to-center and span controls. The process is reversed and repeated to analyze other signals of interest. The disadvantage of this technique is the repeated switching back and forth between wide and narrow spans and the inability to observe both views simultaneously.
In an oscilloscope waveforms are displayed with amplitude of signals on the vertical axis and time on the horizontal axis. Some oscilloscopes utilize a delayed sweep function. Typically, activating a delayed sweep function results in the waveform area being split into two regions. One region is the main sweep. A second region is a delayed sweep, which represents an expansion of the acquired waveform data. A section of the waveform in the main sweep window is highlighted to indicate the part displayed in the delayed sweep window. For example, in Infiniium Oscilloscopes available from Agilent Technologies, having a business address of 395 Page Mill Road, Palo Alto, Calif. 94304, in a delayed sweep mode, the horizontal sweep speed and horizontal position controls change how the waveform is displayed in the delayed sweep window. The sweep speed changes the amount of magnification, while the position changes the part of the waveform in the main sweep window that is displayed in the delayed sweep window. See xe2x80x9cUser""s Quick Start Guidexe2x80x9d for Infiniium Oscilloscopes, available from Agilent Technologies as Publication number 54810-97045, January 2000, p. 3-14.
In accordance with a preferred embodiment of the present invention, a signal analyzer has a display with two regions. In a first region there is displayed a first waveform representing a signal. The first waveform is shown in two dimensions. One dimension represents signal amplitude and one dimension represents signal frequency. In a second region is displayed a second waveform. The second waveform represents a subset of data points of the first waveform. The second waveform also is shown in two dimensions. One dimension represents signal amplitude and one dimension represents signal frequency.
In a preferred embodiment, a box is displayed in the first region. A left edge of the box represents a start point for the second waveform. A right edge of the box represents a stop point for the second waveform. A top edge of the box represents an upper amplitude limit for the second waveform. A bottom edge of the box represents a lower amplitude limit for the second waveform. In response to a user varying any edge of the box, the second waveform displayed in the second region is correspondingly varied.
A control panel includes four controls. A first control is used to position the left edge of the box. A second control is used to position the right edge of the box. A third control is used to position the top edge of the box. A fourth control is used to position the bottom edge of the box. In one embodiment of the present invention, the four controls are multimode so that in a waveform mode the four controls are used to adjust parameters of the first waveform displayed in the first region. The modes are selected with back-lit push buttons. For example, the signal analyzer is a spectrum analyzer. Alternatively, the signal analyzer can be a multi-wavelength meter or any device that analyzes signals and displays the amplitude verses frequency.
The present invention allows a user to observe both a wide and narrow span spectral view simultaneously. The present invention also allows a user to easily select the desired signal from the wide span view to analyze in the narrow span view. The selection technique is much faster and straight-forward than previous select techniques available in spectrum analyzers. Additionally, more information is provided to the user.